1. Field of the Invention
The present invention relates to an electrically conductive roll such as a charging roll, for use in an electrophotographic copying machine, printer, etc.
2. Discussion of Related Art
A charging roll is installed on an electrophotographic copying machine, printer, etc., such that the charging roll is rotated while it is held in pressing contact with an outer circumferential surface of a photosensitive drum, whereby the outer circumferential surface of the photosensitive drum is charged by the charging roll. Described more specifically, the charging roll is used in a roll charging method wherein the photosensitive drum on which an electrostatic latent image is formed is charged by the charging roll. In the roll charging method, the photosensitive drum and the charging roll are rotated such that the charging roll to which a voltage is applied is held in pressing contact with the outer circumferential surface of the photosensitive drum, to thereby charge the outer circumferential surface of the photosensitive drum.
The conductive roll such as the charging roll described above generally includes a suitable center shaft (core metal) as an electrically conductive body, an electrically conductive elastic layer formed on an outer circumferential surface of the center shaft and provided by a rubber layer or a foamed rubber layer, for instance, and a resistance adjusting layer formed on an outer circumferential surface of the conductive elastic layer. The conductive roll further includes, as needed, a protective layer formed on an outer circumferential surface of the resistance adjusting layer.
In the conductive roll constructed as described above, the resistance adjusting layer formed radially outwardly of the conductive elastic layer is conventionally formed of a rubber composition as disclosed in JP-A-11-237782 and JP-A-2000-274424, for instance, which rubber composition includes a rubber material, an electron-conductive agent/agents such as carbon black, an ion-conductive agent/agents such as a quaternary ammonium salt, and an electrically insulating filler/fillers such as silica, in respective suitable amounts. The resistance adjusting layer formed of the rubber composition described above exhibits a suitable degree of electric resistance.
The resistance adjusting layer formed of the rubber composition described above, however, suffers from deterioration of its durability due to an electric current applied thereto during a long use of the roll, in other words, the resistance adjusting layer suffers from an increase in the electric resistance. When the electric resistance is increased up to a level higher than a tolerable or allowable level of a machine on which the conductive roll is installed, an image reproduced by using the conductive roll undesirably suffers from deterioration in the quality due to uneven charging of the photosensitive drum by the conductive roll (due to reduced charging uniformity). For instance, the reproduced image suffers from a multiplicity of sand-like black dots, and the entirety of the image tends to be blackened or darkened.
To prevent deterioration of image quality due to uneven electric resistance, JP-A-2000-284571 proposes a resistance adjusting layer which is formed of a resin composition that includes a plurality of resin materials such as polyolefin. The resistance adjusting layer formed of such a resin composition, however, has a lower degree of resistance to permanent set than the resistance adjusting layer formed of the rubber composition described above. Even if the resistance adjusting layer is formed of a rubber composition which includes a resin such as polyolefin resin, it is difficult to effectively prevent the resistance adjusting layer from being permanently set.